Thursday, October 24, 2013

The spatial patterns of directional phenotypic selection

Alas! Unlike the previous post by Joost, I cannot say that this will
be the next blockbuster Hollywood script. Just a recap of another Ecology
Letters article (Siepielski et
al., 2013). So mundane. I know. Anyways, on to science!

The principal underlying process driving adaptive divergence,
and thus ecological speciation, is selection. Local adaptation occurs among populations,
often in response to directional selection imposed by abiotic and biotic
factors. Since 1983, when Lande and Arnold presented a
standardized method to estimate selection, there have been thousands of studies
that have estimated selection, and recent meta-analyses have looked at how selection varies temporally within populations. However, we lack a comprehensive understanding of the spatial variation in selection among populations that might drive adaptive divergence. Is there actually spatial variation in
selection, or is all such observed variation actually due to sampling error? Ifthere is ‘real’ spatial variation, how does selection vary: in its strength, its direction, or both? Is the variation enough to advance local
adaptation? These questions were the
motivation for our recent article, published in Ecology Letters.

Wordle of the article body. Just because. And wouldn't you know it, the largest word is selection!

My colleagues and I reviewed the literature for studies that
had spatial replicates of selection estimates among at least two populations. We focused on selection on continuous phenotypic traits in un-manipulated
wild populations, and found 60 studies that met our requirements. The first
thing we noticed was a geographical bias in spatially replicated estimates of
selection: the majority of the estimates are in temperate regions of
the northern hemisphere, centred at about 40° latitude.

Figure 1 from the article. Gradients in blue, differentials in red.

Using multivariate models proposed by Morrissey and Hadfield
(2012), we analyzed directional
selection estimates. There is a signature of spatial variation in selection,
even after correcting for sampling error. In other words, after we account for
within-study sampling error, about 12% of the variation in selection we observe is due to real spatial variation in selection among populations.

So spatial variation in selection among populations is real,
but what are the characteristics of this variation? Does it vary more in
direction? Does it vary more in strength? Understanding these types of dynamics
is important, especially when it comes to understanding adaptive divergence. Differences
in the direction of selection among populations are important for two reasons: (1) differences in direction could mean divergent selection among the
populations, and (2) a more rugged fitness landscape could be envisioned in this case,
also important for divergent selection. On the other hand, variation in strength, but not direction, of selection could mean that populations are at various
stages of becoming locally adapted, or could represent some kind of genetic
constraint among populations.

So what did we find? There was variation in the direction of
selection among populations, but where there were differences in direction, the selection estimates were of relatively small magnitude. It appears that
more of the spatial variation observed among populations is in the
magnitude, or strength, of selection, and not in the direction of selection. We
posit three possible reasons for this. First, the selection estimated could be
in response to shared environmental factors among populations. Second,
variation in strength could be present due to different starting population phenotypic
means. In other words, two populations might be subject to the same fitness function, but
if their starting, mean phenotypes are different, then there will be variation in the strength of selection toward whatever local or global optimum is being approached. Lastly, gene
flow could also have an effect. Gene flow can either facilitate or hamper adaption, depending on whether the gene flow is coming from populations of similar or dissimilar
selective regimes; either way, it can affect the strength of selection observed in a population.

One of the things my co-authors and I really wanted to look at
was the spatial structure of selection. Does selection vary in a spatially autocorrelated fashion (such as a gradient in the phenotypic optimum across space), or is it more patchy, or mosaic-like in its
structure? Unfortunately, we could not do a formal analysis for three reasons. The first reason is that there was the lack of spatial replication (the mode number of population replicates was 2). Second, many studies used a small sample size, which produced excessive sampling error for such an analysis. Third, the populations were not always randomly
selected populations, and were instead selected intentionally because of some
kind of contrast among the populations, which would bias our analysis.

The magnitude of spatial variation in selection we found is comparable to that of temporal variation in selection within populations (Siepielski et al., 2009;
Kingsolver et al., 2012; Morrissey & Hadfield, 2012). However, interpreting this comparison must be done with caution for
several reasons. As mentioned above, populations are not always selected
randomly, and this might affect the detection spatial variation more than the detection of temporal
variation in selection. Ideally, we would have
access to several studies that have multiple spatiotemporally replicated selection
estimates, but not a lot of studies do that.

So, folks: that is what we need! More spatiotemporally
replicated studies of selection in naturally occurring populations – particularly studies NOT near 40°N latitude. Don’t forget to report your standard errors, and
if you do gamma estimates, don’t forget to double them (Stinchcombe
et al., 2008)! Happy selection estimating!